This invention relates to optical filters, particularly to narrow-band filters for absorbing optical radiation having a particular selected frequency inside the narrow-band, and for efficiently transmitting optical radiation having frequencies outside the narrow-band but within a broader frequency band surrounding the narrow-band. Typically, the width of the narrow-band is around 5-20 cm.sup.-1 and the width of the broader band is around 200-1000 cm.sup.-1
Certain materials, particularly certain diatomic or polyatomic molecular ions, are known to have what are commonly called matrix isolation spectral bands. When these ions are solved within a crystal of another material, i.e., a solid solvent, they form impurities in the crystal, which is called the "matrix" for the "isolated" molecular ion.
Matrix isolation of molecular ions is useful for analyzing the spectral absorption characteristics of the particular impurity, since spectra of the matrix isolated impurity ion are similar to that of the free molecular ion. Thus, when molecular ion impurities are dissolved in a crystal, analysis of their spectral properties is facilitated, and such analyses have been reported in the literature.
Hitherto the matrix isolation absorption properties of materials in crystals have not been used in connection with filtering of optical radiation.
It is an object of the present invention to provide an optical absorption filter which absorbs optical radiation having a selected frequency, but transmits other radiation within a frequency band which includes the selected frequency.
It is a further object of the present invention to provide such a filter which will pass a high proportion of optical radiation within the frequency band of interest.
It is a further object of the present invention to provide such a filter wherein the absorption frequency of an impurity can be adjusted to coincide with a selected frequency.